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     Basic concepts of the process
The physical concept of growing crystals of predetermined shapes was proposed by the soviet scientist A.V. Stepanov, corresponding member of the USSR Academy of sciences. Back in 1938.
     The Stepanov method (EFG) makes use of various capillary phenomena which allow a liquid column of a predetermined configuration to be formed over the surface of the melt in a crucible by means of special formers.
     A crystal is grown by the Stepanov method (EFG) on a seed crystal from melt with an appropriate choice of temperature conditions and a seed crystal pulling rate. The crystallization front, and hence the crystal being grown, will reproduce the cross – sectional shape of the melt column.

     Equipment for the process
Equipment employed for producing shaped crystals of sapphire includes all basic components of plants used for growing high – melting crystals by the Czochralski method: a sealed working chamber; precision pulling mechanism with a pulling member carrying a seed holder and received in the chamber through its upper flange; automatic process control system; power supply unit; water cooling system; system for evacuating the working chamber and for filling it with an inert gas; control and measurement equipment.
     The main and unique components of the plant for producing sapphire crystals of predetermined shape are the thermal processing zone and the formers.
     A heater surrounds a crucible containing aluminium oxide melt. A former, having vertical supply capillaries and an upper horizontal end platform of a predetermined size, is immersed in the melt. The melt, which wets the former material, rises from the crucible up along the capillaries and spreads over the upper end face of the former so as to form a melt column, the cross – sectional configuration of the column being an exact reproduction of the end face geometry. A crystal is grown upwards on a seed crystal which is oriented in a predetermined crystallographic direction.
     The most important factors of the process are capillary conductions for the formation of a melt column at the end face and temperature distribution in the processing zone.
     The main requirements imposed upon the temperature field in the processing zone are as follows:
   - uniform distribution of temperature over the working end face of the former. With group growing of several crystals simultaneously the temperature of the end faces of all formers should be one and the same;
   - a preset axial temperature gradient in the crystals being grown. This parameter determines a possible pulling rate (hence the output of the process), stability of the crystallization front under temperature disturbances as well as thermal stresses and associated perfection of the crystal structure;
   - minimum overheating of elements of the processing zone, individual portions of the crucible and formers which is only sufficient for preventing spontaneous crystallization of the melt. This ensures an improved quality of the crystals and prolongs the service life of basic components in the processing zone.
     Comprehensive scientific research, process development and design operations have resulted in an optimum design of the thermal processing zone, formers, seed crystal holders and other basic components of the equipment as well as an optimum process for cost – effective manufacture of a wide range of sapphire products.
     The capabilities of the process are based upon:
   - choice of the raw material and its preparation for various applications of sapphire products;
   - growing conditions and subsequent chemical and thermal treatment for producing sapphire products with desired properties;
   - choice of optimum materials for the thermal zone and the formers;
   - effective gas dynamics within the working chamber;
   - long service life of the processing equipment;
   - minimum consumption of energy and cooling water;
   - high output rate for a wide range of sapphire products of a desired quality.



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